In the field of optical multiplexer/demultiplexer or other optical communication devices used in wavelength division multiplexing (WDM) communications, photonic crystals have been drawing attention as a promising material for the development of devices that are higher in performance yet smaller in size and can be manufactured at low costs. A photonic crystal is a dielectric object having an artificial periodic structure. Typically, the periodic structure is created by providing the dielectric body with a periodic arrangement of modified refractive index areas, i.e. the areas whose refractive index differs from that of the body. Within the crystal, the periodic structure creates a band structure with respect to the energy of light and thereby produces an energy region in which the light cannot be propagated. Such an energy region is called the “photonic band gap” or “PBG.” The energy region (or wavelength band) at which the PBG is created depends on the refractive index of the dielectric body and the cycle distance of the periodic structure.
Introducing an appropriate defect into the photonic crystal creates a specific energy level within the PBG (“defect level”), and only a ray of light having a wavelength corresponding to the defect level is allowed to be present in the vicinity of the defect. This means that a photonic crystal having such a defect will function as an optical resonator that resonates with light having a specific wavelength. Furthermore, forming a linear defect will enable the crystal to be used as a waveguide.
As an example of the above-described technique, Patent Document 1 discloses a two-dimensional photonic crystal having a body (slab) provided with a periodic arrangement of modified refractive index areas, in which a linear defect of the periodic arrangement is created to form a waveguide and a point-like defect is created adjacent to the waveguide. This two-dimensional photonic crystal functions as the following two devices: a demultiplexer for extracting a ray of light whose wavelength equals the resonance frequency of the resonator from rays of light having various wavelengths and propagated through the waveguide and for sending the extracted light to the outside; and a multiplexer for introducing the same light from the outside into the waveguide.
Including the one disclosed in Patent Document 1, many two-dimensional photonic crystals are designed so that the PBG becomes effective for either a TE-polarized light, in which the electric field oscillates in the direction parallel to the body, or a TM-polarized light, in which the magnetic field oscillates in the direction parallel to the body. According to this design, if a ray of light containing both kinds of polarized light is introduced into the waveguide or resonator of the two-dimensional photonic crystal, the propagating efficiency of the waveguide deteriorates since one of the two kinds of polarized light diffuses within the body. For example, in a two-dimensional photonic crystal in which the periodic structure has a triangular lattice pattern and each modified refractive index area is circular (or cylindrical), the PBG will be effective for only the TE-polarized light, and a TE-polarized light whose wavelength is within a wavelength range corresponding to the PBG cannot be present within the body. Accordingly, almost no loss of the TE-polarized light will occur at the waveguide or resonator. On the other hand, a TM-polarized light whose wavelength is within the aforementioned wavelength range can escape from the waveguide or resonator into the body and be lost since there is no PGB for the TM-polarized light.
Taking the above problem into account, studies have been conducted on a new design of two-dimensional photonic crystal having a PBG for each of the TE-polarized light and the TM-polarized light in which the two PBGs have a common band. This band is hereinafter called the “complete photonic band gap” or “complete PBG”. For example, Patent Document 2 discloses a two-dimensional photonic crystal in which a complete PBG is created by periodically arranging holes with a C3v-symmetrical shape in a triangular lattice pattern. The C3v symmetry is a symmetry that has an axis of three-fold rotational symmetry and three vertical symmetry planes including the axis. For example, an equilateral triangle is a C3v-symmetrical figure. This two-dimensional photonic crystal prevents any light whose wavelength is included in the complete PBG from leaking from the waveguide, resonator or other element into the body, irrespective of whether the light is TE or TM polarized. Thus, the deterioration of the propagation efficiency of the waveguide is prevented.
In addition, there are two notation system for representing symmetry, i.e. the Hermann-Mauguin notation and Schoenflies notation. The “C3v” symmetry is a Schoenflies notation and can also be expressed as “3 m” using Hermann-Mauguin notation.
Patent Document 3 discloses a two-dimensional photonic crystal with a complete PBG created by periodically arranging holes in a triangular lattice pattern, in which each hole has a C3v-symmetrical shape at a section parallel to the body (“in-plane section”) as in the two-dimensional photonic crystal disclosed in Patent Document 2, and the sectional shape of the hole varies along the thickness direction. As an example of such a hole, Patent Document 3 illustrates a hole whose in-plane sectional shape is an equilateral triangle and whose side length changes along the thickness direction. Another example illustrated in the document is a hole whose in-plane sectional shape is identical except at the upper or lower end portion, at which the hole is filled with the same material as that of the body.
Non-Patent Document 1 discloses a three-dimensional photonic crystal known as “Yablonovite.” Yablonovite is a block-shaped dielectric material with a triangular lattice formed on its surface, in which the hole at each lattice point is extended in three directions (at 120° intervals) with an angle of 35° from the surface normal. This structure has a diffraction index that is periodically distributed not only within the plane parallel to the surface of the dielectric block, but also along the depth of the dielectric block. As a result, a complete PBG is created in any direction within the crystal. This PBG is a complete PBG that does not depend on the polarizing direction.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-272555 (Paragraphs [0023] to [0027] and [0032]; FIGS. 1, 5 and 6)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-099672 (Paragraphs [0008], [0034], [0040], [0054] and [0055]; FIGS. 1 and 10 to 15)
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2006-065150 (Paragraphs [0019], [0020] and [0036]; FIGS. 1 and 3)
Non Patent Document 1: E. Yablonovitch et al. “Photonic band structure: The face-centered-cubic case employing nonspherical atoms”, Physical Review Letters 67 (1991) 2295-2298.